1. Field of the Invention
The present invention relates to a method of increasing the silica to metal oxide ratio in alkali metal silicate solutions by changing the distribution of the component silicate anions in solution. More particularly, the present invention relates to a method of increasing the silica to metal oxide ratio in alkali metal silicate solutions containing smaller and larger anions therein by removing the smaller size anions using a strong cation exchange resin to condense some of the smaller size anions as silica in the pores of the strong cation exchanger resin and leaving the larger, more siliceous anions in the solution.
2. Reported Developments
Alkali metal silicate solutions have many technical applications including their application: as binders in coatings; as thickening agents and stabilizers in paints; as adhesives; as special cements; as raw materials for the production of silica sols and gels; and as components in vehicles for pharmaceutical and cosmetic compositions. Furthermore, sodium silicates are used as cleaners and detergents that require a controlled alkalinity. Sodium silicate solutions with molar SiO.sub.2 :Na.sub.2 O ratios less than 2.5, especially meta- and orthosilicates, are most commonly used in cleaners and detergent products.
When used as binders, adhesives and deflocculants, which depend on the presence of polysilicate ions, sodium silicates having an SiO.sub.2 :Na.sub.2 O molar ratio of 2.5 to 3.8 are required. For maximum adhesive strength the lower ratio types are used because they can be obtained with a higher concentration of solids. For more water resistant bonds, the higher ratios are preferred.
For the production of precipitated silicas, sols, and gels, sodium silicates of molar ratios of about 3.3 are used rather than the lower ratios, since less acid is required for neutralization of alkali per unit of silica, and this ratio is available at low cost because such large volumes are produced and because the alkali component is more expensive than the siliceous component. If, on the other hand, acid consumption is an important factor, higher ratios may be used to advantage.
The properties of alkali metal silicates, such as viscosity, concentration, adhesive strength, rate of solidification, solubility in water, and stability of the solution are closely related and are largely controlled by the variables of the alkali metal used, the ratio of SiO.sub.2 to alkali metal oxide expressed by either weight or molar ratio, and the concentration of solids in the solution. Sodium silicates are produced as glasses having SiO.sub.2 :Na.sub.2 O molar ratios of 1.6-3.9. These are sold in lump or pulverized forms and as partly hydrated powders and concentrated solutions. For the production of colloidal and finely divided silica powders the 3.25 ratio sodium silicate solution is predominantly used.
The maximum practical silica concentration in commercial sodium silicate solutions is limited by the viscosity which must be low enough for the solutions to be poured under ordinary conditions. The higher the SiO.sub.2 :Na.sub.2 O ratio, the lower the maximum silica concentration in commercial products. The composition of typical commercial alkali metal silicates are shown in Table I.
TABLE I __________________________________________________________________________ Alkali Wt. Ratio Molar Ratio Na.sub.2 O Density Viscosity Metal SiO.sub.2 :M.sub.2 O SiO.sub.2 :M.sub.2 O SiO.sub.2 (%) (%) (lb/gal) (centipoise) __________________________________________________________________________ Sodium 3.75 3.87 25.3 6.75 11.0 220 3.25 3.36 29.9 9.22 11.8 830 3.25 3.36 28.4 8.7 11.6 160 3.22 3.33 27.7 8.6 11.5 100 2.87 2.97 32.0 11.1 12.4 1,250 2.58 2.67 32.1 12.5 12.6 780 2.50 2.58 26.5 10.6 11.7 60 2.40 2.48 33.2 13.85 13.0 2,100 2.20 2.27 29.2 13.3 12.5 -- 2.00 2.07 29.4 14.7 12.8 400 2.00 2.07 36.0 18.0 14.1 70,000 1.90 1.96 28.5 15.0 12.7 -- 1.80 1.86 24.1 13.4 12.0 60 1.60 1.65 31.5 19.7 14.0 7,000 Potassium 2.50 3.92 20.8 8.3 10.5 40 2.20 3.45 19.9 9.05 10.5 7 2.10 3.29 26.3 12.5 11.5 1,050 __________________________________________________________________________
The prior art has spent a great deal of effort to provide alkali metal silicates of desired SiO.sub.2 :Na.sub.2 O molar ratios and solid contents. While the selection of a particular alkali metal oxide is rather easy, the provision of proper ratios and solid contents proved to be difficult requiring various theories and approaches to design methods for obtaining them. The difficulty lies in the nature of sodium silicates in water including solubility, viscosity, rate of solidification, and stability which are controlled by the ratio of SiO.sub.2 :Na.sub.2 O; relatively small changes in the ratio result in unpredicted and/or undesirable properties.
Solutions with a low mole ratio, that is up to about 2 SiO.sub.2 :M.sub.2 O, where M is sodium or potassium, are very stable and have a low viscosity even at fairly high concentrations, but when they are used as binders or adhesives they set slowly and the bonds have little resistance to weathering, and therefore are not acceptable in such technical applications. Alkali metal silicate solutions having a mole ratio between 2 and 4 SiO.sub.2 :M.sub.2 O set more rapidly and the bond is less readily soluble but they do have undesirably high viscosities and low stabilities especially in the higher ranges above 3.5. It is to be expected that the setting rate, bond strength, and the resistance to weather should increase as the mole ratio of silica to metal oxide is increased above 4. Unfortunately, such solutions are so unstable and so viscous as to be impractical either to manufacture by ordinary means or to employ in technical applications.
In many of these technical applications it is necessary to have solutions of alkali metal silicate at high concentrations with good adhesive properties, rapid setting rates, and bonds which are resistant to re-solution by water, and at the same time the alkali silicate solution should have a low viscosity and a good stability at ambient temperatures over long periods of time.
Typical examples of the prior art approaches to provide alkali metal silicate solutions with desired ratios of SiO.sub.2 : metal oxide and solid contents follow.
U.S. Pat. No. 3,113,112 relates to a process for the preparation of stable aqueous silica solutions comprising: mixing an aqueous solution of a soluble silicate with a weakly acidic cation exchange resin in the hydrogen form which has been adjusted to a pH between 6 and 8 so as to remove most of the alkali; and treating the mixture by centrifugation, filtration or decantation to separate the solution from the cation exchanger. The solution is concentrated by means of evaporation and its pH is adjusted to between 9.5 to 10.5 to insure stability during storage.
U.S. Pat. No. 3,625,722 pertains to a process for preparing stable alkali metal silicate solutions having silica contents of from 10 to 35% and ratios ranging between 4:1 and 12:1 SiO.sub.2 : alkali metal oxide. Stability is obtained by incorporating a small amount of a quaternary ammonium compound.
U.S. Pat. No. 3,492,137 discloses siliceous compositions containing 10 to 35% by weight silica and sodium oxide and having a silica to sodium oxide weight ratio of 4.0:1 to 6.0:1, made by mixing colloidal amorphous silica with a sodium silicate solution and heating the mixture between 40.degree. to 100.degree. C.
U.S. Pat. No. 3,533,816 relates to a method of making high ratio alkali metal silicates comprising: rapidly mixing a silicic acid solution with an alkali metal hydroxide or silicate solution at room temperature to give a solution having a SiO.sub.2 :M2O mole ratio of 4 to 6 or 8 to 40; and concentrating the solution by vacuum evaporation at a temperature up to 45.degree. C. The silicate solution must have a SiO.sub.2 concentration of less than 3.5% w/w since more concentrated solutions gel.
I have now discovered a method by which a high silica to metal oxide ratio in alkali metal solutions can be achieved, as well as the concentration of alkali metal solutions increased without the drawbacks of costly and cumbersome procedures, precipitation and gelation. As contrasted to the prior art using specific starting materials limited by pH and particle size to achieve the desired end products, the present invention uses a variety of commercially available water soluble alkali metal silicates.